A colour proof of a colour image is required for inspection and approval by the printer or his client before the colour image is printed on the production printing press. There are two important reasons to create a proof before printing a given job:                to check for colour accuracy and resolution;        to confirm that layout, fonts and other design elements were not lost or deformed by the time the image data reached the output stage.        
Very good colour quality of the colour proof is absolutely required.
To make a traditional off-press proof, colour separations of the image (e.g. a cyan, a magenta, a yellow and a black separation) are produced on a substrate which may be photographic film; these colour separations are then used to make the colour proof of the image.
In digital colour proofing, the digital data that represent the colour image are directly sent to a high resolution, high-quality printer that prints the colour proof on a receiving substrate, thus omitting the intermediate step of producing colour separations on a substrate.
Ink-jet printers may be used for digital colour proofing. The Lexmark Optra™ Color 45 ink-jet printer has two application stations for applying cyan ink to the receiving substrate, two stations for magenta ink, one for yellow and one for black ink, which makes a total of six application stations.
When using existing ink-jet printers for colour proofing, the problem is that the quality of the proof, with respect to colour, is not good enough. The insufficient colour quality includes unsatisfactory colour stability and too little or not well spaced colour levels. Moreover, a colour cast may occur in the image and especially in the wedges that are customarily printed near the edges of the proof; colour cast is especially disturbing in neutral or achromatic, i.e. greyish, wedges.
Patent application EP-A-0 388 978 discloses ink-jet printers that use in a first embodiment three black inks but no ink of another colour, and in a second embodiment two cyan, two magenta, two yellow and two black inks. The ink tanks are integrated in the ink-jet head cartridge to provide advantages such as a simple control circuit and more convenience with respect to replacement of the head cartridge; the inks of the same colour are light and dark inks. These ink-jet printers do not offer satisfactory colour quality for proofing purposes.